In recent years, a pedestrian-to-vehicle communication system that carries out radio communications between a communication device for pedestrians which is owned by a pedestrian and a vehicle-mounted communication device mounted in a vehicle, and estimates and judges the risk of collisions, etc. by enabling them to notify their respective positions and information to each other in order to protect the pedestrian from traffic accidents has been researched and developed. A problem arising in such a system is that because a vehicle-mounted communication device mounted in a vehicle and a communication device for pedestrians which a pedestrian carries exchange pieces of information including their respective positions, speeds, directions, etc. between them frequently, when the surrounding area is crowded with vehicles and pedestrians, the communication traffic increases, and hence information does not reach its destinations or reaches its destinations long behind. When no information from pedestrians reaches the vehicle, the vehicle-mounted communication device cannot determine the possibility of collisions between pedestrians and the vehicle, and this may lead to a traffic accident.
Although no prior art documents about a pedestrian-to-vehicle communication system that deals with such a problem have been found, prior art documents relating to a vehicle-to-vehicle communication system that communicates between vehicles have been found. For example, patent reference 1 discloses a vehicle-to-vehicle communication system that recognizes vehicles which carry their respective vehicle-to-vehicle communication devices and which are travelling toward an identical intersection to form a vehicle group, and that by shortening the length of each of time intervals at which each of the vehicles at the head and tail of the vehicle group transmits data and lengthening the length of each of time intervals at which each of the other vehicles included in the vehicle group transmits data, can notify the existence of the vehicle group to other vehicles approaching the vehicle group at an early time and prevent the communication traffic from increasing between vehicles which construct the vehicle group.